Surface acoustic wave filters consist generally of an input and output transducer located on a piezo-electric material substrate. The transducers each consist of a pair of comb-like structures in which the teeth of a pair are interleaved. The size and spacing of the teeth of the comb-like structure and the number thereof determine the frequency response passband of the particular transducer. The teeth or electrodes of the comb-like structures forming the transducer are formed on the substrate material using a photolithographic process. For example, the substrate is coated with a conductive material and, by means of a photographic process, a mask is generated which is placed over the material coated on the substrate such that the exposed coating can be etched away leaving the comb-like structure of the mask. The artwork generator for making the masks has certain limitations in that the artwork can only be generated in certain size steps, each of which produce a different frequency passband.
In the prior art the center frequency of the passband or the characteristic frequency response of the transducer can be determined by determining the width and the spacing involved in the transducers during the manufacturing process. Characteristic frequencies intermediate the artwork generator resolution cannot be obtained.
It is a general object of the present invention to provide an improved surface acoustic wave device in which the frequency passband can be adjusted to have a center frequency which lies between the characteristic frequencies of the input and output transducers.
It is another object of the present invention to provide an improved surface acoustic wave device having a frequency response passband about a center frequency which is beyond the resolution of the artwork generator used in manufacturing the input and output transducers.